1. Field of the Invention
This invention relates to a torque detector which detects torque by converting the torsion of a mechanism for producing an angular displacement, such as a torsion bar, into a change of voltage and more particularly, to the type using a strain gage.
2. Description of the Prior Art
Up to now a number of torque detectors have been disclosed, these being classified roughly into three types.
The first is the magnetic type torque detector which detects a magnetic change produced on a torsion bar by application of torque. This type of torque detector is disclosed, for example, in U.S. Pat. No. 4,506,554. Although the magnetic type torque detector can detect a torque applied to the torsion bar by the use of a magnetic sensor such as a coil without making contact with the torsion bar, it has the problem that an electric circuit becomes complicated.
The second is the electrostatic type torque detector which detects torque by converting an angular displacement produced on the torsion bar by application of torque into a change of electrostatic capacity. This type of torque detector is disclosed, for example, in U.S. Pat. No. 4,522,278. Although the electrostatic type torque detector can detect a torque applied to the torsion bar, a change of electrostatic capacity is generally small, so that there is the problem that it is not only in need of a precise electronic circuit, but also apt to be influenced by a change of surroundings such as humidity.
The third is the resistance type torque detector which detects torque by converting an angular displacement produced on the torsion bary by application of torque into a change of electric resistance. Torque detectors of this type are further classified roughly into two types: one using a potentiometer and the other using a strain gage. The torque detector using the potentiometer is disclosed, for example, in Japanese Patent Laid-Open No. 58-177773. The torque detector using the strain gage is disclosed, for example, in Japanese Patent Laid-Open No. 59-2099644. The resistance type torque detector has the problem that the durability is poor because the potentionmeter or strain gage causes a secular change.